The presently disclosed embodiments are directed to flexible imaging members in electrostatography. More particularly, the embodiments pertain to an improved curl-free flexible electrophotographic imaging member prepared to give an imaging member resistant to chemical contaminants, photoelectrically stable, and having a simplified material structure without the need of an anticurl back coating layer, and a process for making and using the member.
In electrophotographic or electrostatographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. Typical flexible electrostatographic imaging members include, for example: (1) electrophotographic imaging member belts (belt photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging member belts for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring belt which is used to remove the toner images from a photoreceptor surface and then transfer the very images onto a receiving paper. The flexible electrostatographic imaging members may be seamless or seamed belts; and seamed belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam.
Typical electrophotographic imaging member belts include a charge transport layer and a charge generating layer on one side of a supporting substrate layer and an anticurl back coating coated onto the opposite side of the substrate layer. A typical electrographic imaging member belt includes a dielectric imaging layer on one side of a supporting substrate and an anticurl back coating on the opposite side of the substrate to render flatness. Although the scope of the present embodiments covers the preparation of all types of flexible electrostatographic imaging members, for reasons of simplicity, the discussion hereinafter will focus only on flexible electrophotographic imaging members.
Electrophotographic flexible imaging members may include a photoconductive layer including a single layer or composite layers. Since typical flexible electrophotographic imaging members exhibit undesirable upward imaging member curling, an anti-curl back coating, applied to the backside, is required to balance the curl. Thus, the application of anticurl back coating is necessary to provide the appropriate imaging member belt with desirable flatness.
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous charge transport layer and the supporting conductive layer. Alternatively, the charge transport layer may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a receiving member such as paper.
In the case where the charge generating layer is sandwiched between the outermost exposed charge transport layer and the electrically conducting layer, the outer surface of the charge transport layer is charged negatively and the conductive layer is charged positively. The charge generating layer then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the charge transport layer. In the alternate case when the charge transport layer is sandwiched between the charge generating layer and the conductive layer, the outer surface of the charge generating layer is charged positively while conductive layer is charged negatively and the holes are injected through from the charge generating layer to the charge transport layer. The charge transport layer should be able to transport the holes with as little trapping of charge as possible. In flexible imaging member belt such as photoreceptor, the charge conductive layer may be a thin coating of metal on a flexible substrate support layer.
Typical negatively-charged electrophotographic imaging member belts, such as flexible photoreceptor belt designs, are made of multiple layers including a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer. The charge transport layer is usually located at the outermost layer. The charge transport layer is generally coated and dried at elevated temperatures (e.g., about 120° C.), and then cooled down to ambient room temperatures. Normally, an upward curling of the multilayered photoreceptor is observed during the manufacturing of the web stock of coated multilayered photoreceptor materials, which is a consequence of thermal contraction mismatch between the charge transport layer and the substrate support due to the heating/cooling processing step. According to the mechanism: (a) as the web stock carrying the wet applied charge transport layer is dried at elevated temperature, dimensional contraction does occur when the wet charge transport layer coating is losing its solvent during 120° C. elevated temperature drying, but at 120° C. the charge transport layer remains as a viscous flowing liquid after losing its solvent. Since its glass transition temperature (Tg) is at 85° C., the charge transport layer after losing of solvent will flow to re-adjust itself, release internal stress, and maintain its dimension stability; (b) as the charge transport layer now in the viscous liquid state is cooling down further and reaching its glass transition temperature (Tg) at 85° C., the CTL instantaneously solidifies and adheres to the charge generating layer because it has then transformed itself from being a viscous liquid into a solid layer at its Tg; and (c) eventual cooling down the solid charge transport layer of the imaging member web from 85° C. down to 25° C. room ambient will then cause the charge transport layer to contract more than the substrate support since it has about 3.7 times greater thermal coefficient of dimensional contraction than that of the substrate support. This differential in dimensional contraction results in tension strain built-up in the charge transport layer which therefore, at this instant, pulls the imaging member upward to exhibit curling. If unrestrained at this point, the imaging member web stock will spontaneously curl upwardly into a 1.5-inch tube. To offset the curling, an anticurl back coating is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer, and render the imaging member web stock with desired flatness.
Curling of an electrophotographic imaging member web is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a belt. An anticurl back coating, having an equal counter curling effect but in the opposite direction to the applied imaging layer(s), is applied to the reverse side of substrate support of the active imaging member to balance the curl caused by the mismatch of the thermal contraction coefficient between the substrate and the charge transport layer, resulting in greater charge transport layer dimensional shrinkage than that of the substrate. Although the application of an anticurl back coating is effective to counter and remove the curl, nonetheless the resulting imaging member in flat configuration does tension the charge transport layer creating an internal build-in strain of about 0.27% in the layer. The magnitude of CTL internal build-in strain is very undesirable, because it is additive to the induced bending strain of an imaging member belt as the belt bends and flexes over each belt support roller during dynamic fatigue belt cyclic motion under a normal machine electrophotographic imaging function condition in the field. The summation of the internal strain and the cumulative fatigue bending strain sustained in the charge transport layer has been found to exacerbate the early onset of charge transport layer cracking, preventing the belt to reach its targeted functional imaging life. Moreover, imaging member belt employing an anticurl backing coating has additional total belt thickness to thereby increase charge transport layer bending strain as it flexes over each belt support rollers and speed up belt cycling fatigue charge transport layer cracking. The cracks formed in the charge transport layer as a result of dynamic belt fatiguing are found to manifest themselves into copy print-out defects, which thereby adversely affect the image quality on the receiving paper.
Curling has a further undesirable impact under a normal imaging belt machine functioning condition, because different segments of the imaging surface of the photoconductive member are located at different distances from charging devices, causing non-uniform charging for proper latent image formation. Therefore, developer applicators and the like, during the electrophotographic imaging process, may all adversely affect the quality of the ultimate developed images in the printout copy. For example, non-uniform charging distances can manifest as variations in high background deposits during development of electrostatic latent images near the edges of paper. Since the anticurl back coating is an outermost exposed backing layer and has high surface contact friction when it slides and flexes over the machine subsystems of the belt support module, such as rollers, stationary belt guiding components, and backer bars, during dynamic belt cyclic function, these mechanical sliding interactions against the belt support module components not only exacerbate anticurl back coating wear to lose its anti-curling control capability to result in imaging member belt curling-up problem, it does also generate of debris/dirt which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance. Moreover, anticurl back coating abrasion/scratch damage does also produce unbalance forces generation between the charge transport layer and the anticurl back coating to cause micro belt ripples formation during electrophotographic imaging processes, resulting in streak line print defects in output copies to deleteriously impact image printout quality and shorten the imaging member belt functional life.
Undesirably, high contact friction of the anticurl back coating against machine subsystems is further seen to cause the development of tribo-electrostatic charge built-up problem. In other machines the electrostatic charge builds up due to contact friction between the anti-curl layer and the backer bars increases the friction and thus requires higher torque to pull the belts. In full color machines with 10 pitches this can be extremely high due to large number of backer bars used. At times, one has to use two drive rollers rather than one which are to be coordinated electronically precisely to prevent any possibility of sagging. Static charge built-up in anticurl back coating by frictional action on the anticurl back coating increases belt drive torque, in some instances, has also been found to result in absolute belt stalling. In other cases, the electrostatic charge build up can be so high as to cause sparking.
Another problem encountered in the conventional belt photoreceptors using a bisphenol A polycarbonate anticurl back coating that are extensively cycled in precision electrostatographic imaging machines utilizing belt supporting backer bars, is an audible squeaky sound generated due to high contact friction interaction between the anticurl back coating and the backer bars. Further, cumulative deposition of anticurl back coating wear debris onto the backer bars may give rise to undesirable defect print marks formed on copies because each debris deposit become a surface protrusion point on the backer bar and locally forces the imaging member belt upwardly to interferes with the toner image development process. On other occasions, the anticurl back coating wear debris accumulation on the backer bars does gradually increase the dynamic contact friction between these two interacting surfaces of anticurl back coating and backer bar, interfering with the duty cycle of the driving motor to a point where the motor eventually stalls and belt cycling prematurely ceases. Additionally, it is important to point out that electrophotographic imaging member belts that required anticurl back coating to provide flatness have more than above list of problems, they do indeed incur additional material and labor cost impact to imaging members' production process.
Since the charge transport layer in the negatively charged flexible imaging member is the top outermost exposed layer, it is constantly subjected to chemical vapor contaminants exposure/interaction during electrophotographic imaging process in the field to negatively impact function. For example, exposure to the vapor amine species (from ammonia) emitted from common house cleaning agents have been seen to interact with the imaging member charge transport layer, causing material degradation to promote pre-mature onset of charge transport layer cracking and exacerbation of wear failure which severely cut short the functional life of the imaging member. In one particular instant, amine vapor impact on copy printout quality degradation has recently been seen when pre-printed papers (papers having pre-printed images which employed amine agents catalyzed UV cured ink) are used by customers for subsequent addition of xerographic images over the pre-printed paper blank spaces; that is the accumulation of amine residues deposition onto the imaging member charge transport layer surface, after repeatedly making contact with receiving papers during xerographic imaging process, is found to cause ghosting image defects print-out in the output copies to severely degrade copy quality. Since ghosting image defects in the output copies are unacceptable print quality failures, so it does require immediate termination of belt function for replacement.
Moreover, the imaging members do also exhibit typical mechanical failures such as charge transport layer cracking when functioned under the normal machine electrophotographic imaging service conditions. The frequently seen charge transport layer cracking failure is unique in belt members, as it is induced by dynamic fatigue belt flexing over the supporting rollers of a machine belt support module and exacerbated by exposure to ozone in the corona species emitted by machine charging devices since ozone attack the polymer binder in the charge transport layer causing polymer chain scission and fragmentation.
In addition, for typical imaging members of the conventional designs, they do all have an inherent photoelectrical instability shortfall, such as for example, the monotonously increase in electrical property cycle-up during electrophotographic imaging process has always been found to be a critical issue, because this electrical cycle-up limits the imaging members practical application value as it shortens the imaging member's functional life in the field.
The above mentioned shortcomings and failures pre-maturely cut short the imaging member function and prevent the imaging member from reaching the intended full service life and require frequent costly replacement in the field. A practical solution to resolve all the issues is therefore urgently needed for achieving imaging member production quality and cost delivery objective.